Operation of the magnet of a magnetic resonance imaging (MRI) system

ABSTRACT

Some implementations provide a method for safe operation of a magnetic resonance imaging (MRI) system, the method including: determining, at least in part by using a sensor device, location information that indicates a location of an MR-incompatible object relative to the MRI system, the MRI system generating a polarizing magnetic field for imaging a subject; based on the determined location information, determining, by a control unit associated with the MRI system, that the MR-incompatible object poses an operational hazard to the MRI system; and in response to determining that the MR-incompatible object poses an operational hazard to the MRI system, reducing, by the control unit, a strength of the polarizing magnetic field.

BACKGROUND

The present disclosure relates to magnetic resonance imaging and, inparticular, operation of the magnet of a magnetic resonance imaging(MRI) system.

SUMMARY

In one aspect, some implementations provide a method for safe operationof a magnetic resonance imaging (MRI) system, the method including:determining, at least in part by using a sensor device, locationinformation that indicates a location of an MR-incompatible objectrelative to the MRI system, the MRI system generating a polarizingmagnetic field for imaging a subject; based on the determined locationinformation, determining, by a control unit associated with the MRIsystem, that the MR-incompatible object poses an operational hazard tothe MRI system; and in response to determining that the MR-incompatibleobject poses an operational hazard to the MRI system, reducing astrength of the polarizing magnetic field.

Implementations may include one or more of the following features. Thepolarizing magnetic field may be substantially uniform. Determining thatthe MR-incompatible object poses an operational hazard to the MRI systemmay include determining that the MR-incompatible object is presentwithin a safety zone of a main magnet of the MRI system. Determiningthat the MR-incompatible object poses an operational hazard to the MRIsystem may include determining that the MR-incompatible object isaccelerating towards a main magnet of the MRI system. The sensor devicecomprises a camera and determining the location information may includeperforming image processing on images obtained from the camera to locatethe MR-incompatible object relative to the MRI system. The sensor devicemay include one of: a Gauss meter, a Hall probe, a magnetic field probe,or an optical tracker. The sensor device may be mounted on theMR-incompatible object.

Determining the location information may include detecting a change in acurrent that powers a main magnet of the MRI system, the change beinginduced by a motion of the MR-incompatible object within the polarizingmagnetic field. Reducing the strength of the polarizing magnetic fieldmay include initiating a shutdown of a main magnet of the MRI system.

The method may further include ramping up the strength of the polarizingmagnet field when the operational hazard to the MRI system posed by theMR-incompatible object no longer exists.

In another aspect, some implementations provide a magnetic resonanceimaging (MRI) system, the MRI system including: a housing having a borein which a subject to be imaged is placed; a main magnet accommodated bysaid housing and configured to generate a polarizing magnet field withthe bore; pulse generating coils configured to generate and apply radiofrequency (RF) pulses in sequence to scan the subject; gradient coilsconfigured to provide perturbations to the polarizing magnet field thatencode MRI signals acquired in response to the applied RF pulses; and acontrol unit coupled to the main magnet and configured to: accesslocation information that indicates a location of an MR-incompatibleobject relative to the MRI system; based on the location information,determine whether the MR-incompatible object poses an operational hazardto the MRI system; and in response to determining that theMR-incompatible object poses an operational hazard to the MRI system,reduce a strength of the polarizing magnetic field.

Implementations may include one or more of the following features. Thepolarizing magnetic field may be substantially uniform. The control unitmay be configured to receive location information of the MR-incompatibleobject from a sensor mounted on the MR-incompatible object. The sensormay include one of: a Gauss meter, a Hall probe, a magnetic field probe,or an optical tracker.

The MRI system may further include a camera, wherein the control unit isconfigured to process images from the camera to determine the locationinformation of the MR-incompatible object.

The MRI system may further include a current sensor, wherein the controlunit is configured to process a signal from the current sensor to detecta change in a current that powers the main magnet, the change beinginduced by a motion of the MR-incompatible object within the polarizingmagnetic field.

To reduce the strength of the polarizing magnetic field, the controlunit may be configured to initiate a shutdown of the main magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the drawings, in which:

FIG. 1A shows a perspective view of an example of a magnetic resonanceimaging (MRI) system configured for safe shutdown.

FIG. 1B shows a profile view of the MRI system configured for safeshutdown.

FIG. 2 illustrates an example of an MRI systems configured for safeshutdown.

FIG. 3 is a flow chart showing an example of a process for automaticsafe shutdown of a magnet of an MRI system.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

Magnetic resonance imaging (MRI) systems generally include a main magnetgenerating a magnetic field tens of thousands times stronger than theearth's magnetic field. As a result, in certain situations, the mainmagnet can attract MR-incompatible objects. For example, foreign metalobjects, when brought within the vicinity of the magnetic field, canbecome projectiles flying into the main magnet and causing disastrousdamage. In this example, MR-incompatible objects, such as an objectincluding ferromagnetic material, can be attracted by the fringe fieldof the main magnet. The MR-incompatible objects can include carts,oxygen tanks, and tools (such as wrenches). Once attracted, theseobjects may be accelerated by the magnetic force imparted by the mainmagnetic field and can become high-velocity projectiles capable ofdamaging the magnet or harming a human patient. This is a significantoperational hazard of the MRI system, especially when a patient is beingscanned inside the magnet.

Some implementations of the present disclosure may include a detectionmechanism for the MRI system to identify MR-incompatible objects withina safety zone of a magnet. The identification may classify the objectsaccording to the corresponding operational hazards. If theMR-incompatible object is within a specific region (e.g., the fringefield of the system), then the MRI system may react by automaticallyinitiating a protection sequence. In some instances, the MRI system mayinitiate an automatic winding down of the main magnetic field. Forexample, the MRI system may decrease the current through thesuperconducting wires to reduce the magnetic field or to completely shutoff the field. In some instances where a patient is placed in themagnet's bore, the protection sequence may include deploying aprotective physical barrier, such as airbag(s), to shield the patientfrom projectiles while the current (and thus magnetic field) is beingshut down.

Referring now to FIG. 1A, an example of a magnetic resonance imaging(MRI) system 100 is shown in which a magnet housing 105 is placed on abase 109. Base 109 may include a portable cart, as shown. In someinstallations, base 109 may be affixed to the floor of the scanningroom. Magnet housing 105 includes a solenoid magnet and bore area 101,where a human patient may be placed to be scanned. The solenoid magnetmay be generally known as the main magnet. The solenoid magnet maygenerate a substantially uniform magnetic field for imaging the humanpatient placed inside bore area 101. This magnetic field may generallyserve as a static polarizing field.

Referring to FIG. 1B, patient 103 can be placed in bore area 101. Inthis example, patient head area 102 is placed inside the magnetic fieldto be imaged by coil assembly 104. As shown in FIGS. 1A and 1B, coilassembly 104 is shaped as an annular structure and housed within theinner bore of solenoid magnet. In this example, coil assembly 104includes a gradient coil 104A and an RF coil 104B. The gradient coil104A may generate a perturbation of the static polarizing field toencode magnetizations within the human patient's body. In someconfigurations, coil assembly 104 may include a radio frequency (RF)coil 104B to transmit RF pulses as excitation pulses. The RF coil 104Bmay also be configured to receive MR signals from the human patient inresponse to the RF pulses. In some instances, housing 105 may includeseparate receive coils to receive the MR signals from the human patient.In these instances, radio-frequency (RF) signals are, for example,received by local coils for imaging a subject. In one example, a headcoil in a birdcage configuration is used for both transmitting andreceiving RF signals for imaging the subject's head area 102. In anotherinstance, RF coil 104B is used for transmitting an RF signal into thesubject and a phased array coil configuration is used for receiving MRsignals in response.

Referring now to FIG. 2, a safety zone 201 may be prescribed insidescanning room 200. Safety zone 201 may represent a region in which thepresence of suspected MR-incompatible objects would trigger a safeshutdown of the magnet. In some instances, safety zone 201 extendsoutside the fringe field (defined by, e.g., the 5 Gauss line) of themagnet. In other instances, safety zone 201 may cover the same area asthe fringe field (defined by, e.g., the 5 Gauss line) of the magnet. Instill other instances, safety zone 201 may cover an area within the5-Gauss line perimeter, for example, a 10-Gauss line. As disclosedherein, MR-incompatible objects refer to objects that pose anoperational hazard to the MRI system. Examples include ferromagneticobjects such as ferromagnetic objects. Magnet housing 105 includessolenoid magnet and bore area 101, where a human patient 203 may beplaced to have an MRI of his or her head region 202. In thisillustration, human patient 203 is placed on patient bed 204 that isslide-able into bore area 101. In this example, cart 210 is locatedinside safety zone 201. The cart 210 may be brought into the scanningroom when patient 203 was transported in. The cart 210 may includeMR-incompatible objects, for example, oxygen tanks (including metalcover) and mechanical tools (such as wrenches).

Such objects may pose an operational hazard because these objects aretypically magnetic and can become projectiles flying into magnet housing105, damaging the magnet or even patient 203 who may be undergoing anMRI procedure in bore 101. Some implementations incorporate a detectionsystem 206 including, for example, a control unit and one or more sensordevices. The one or more sensor devices are configured to determinelocation information that indicates the location of such an objectrelative to MRI system 100. Based on this information, the control unitcan determine if the object poses an operational hazard, for example,because it is inside safety zone 201. If the control unit determines theobject poses an operational hazard, the control unit may take action,such as causing a reduction of the field strength of the magnet. In someinstances, one or more airbags (as an example of a physical barrier) maybe inflated to buffer the potential impact from a possible projectile.

The detection system may employ one or more detection mechanisms toidentify the operational hazard in the scan room. As explained below,examples of the detection mechanism include an optical approach, aradio-frequency (RF) approach, a magnetic approach, or an inductiveapproach. The optical approach may employ sensor devices such as one ormore camera devices or even infrared (IR) sensors. The RF approach mayutilize sensor devices that include antenna devices for transmitting andreceiving RF waves. The magnetic approach may employ sensor devices thatleverage the use of magnetic field probes while the inductive approachmay employ a sensor device that senses the inductive effect on thecurrent powering the magnet caused by magnetic objects in the field ofthe magnet.

As noted above, in some instances, the detection system 206 may includea camera device. The camera device may operate in visible light orinfrared ranges. In one example, the camera device may be mounted on themagnetic housing 105. In another embodiment, the camera device may belocated outside the room, for example, in the console room, to monitorthe scan room through a glass shield. As an illustration, the cameradevice can capture video frames and then process the captured videoframes to identify potentially MR-incompatible objects and track themotions of the identified objects. Examples of such objects includeanesthesia carts, oxygen tanks, or working tools such as wrenches. Thereadout from the camera device, for example, a live video feed, mayserve as one source of input data for further processing by thedetection system 206, as discussed further below.

In some cases, the detection system 206 may include a system thatleverages the readout from optical or electromagnetic (EM) trackingmarkers placed on potentially MR-incompatible objects. Examples ofoptical tracking markers include fluorescent objects, orlight-emitting-diodes. For illustration, the system may monitor thepositional information of such objects in the scan room by sensing thewhereabouts of optical or EM tracking markers and projecting thepositional information on a live video stream of the scan room.Generally, the sensed positional information can be another source ofinput data for further processing by the detection system 206, asdiscussed further below.

In some instances, the detection system 206 may include a laserprojector device to emit, for example, infrared (IR) beams. In oneillustrative example, the laser projection device rotates a transmittingIR beam to scan the surroundings. In these instances, photosensors (suchas photodiodes) may be placed around the perimeter inside the scan roomto detect any blockage of projected laser beam from laser projector 206.For example, laser sensors may be placed on the floor along theperimeter of the safety zone 201, or along the shoe molding level of thesidewalls of the scan room. Some implementations of laser projector mayfurther include photosensors (such as photodiode devices) to receivelaser reflections due to blocking objects. The readout may becommunicated to a control unit of MRI system 100, which controls thepower supply of the magnet. Such readout may generally serve as yetanother source of input data for further processing by the detectionsystem 206, as discussed further below.

The detection system 206 may also operate in radio frequency (RF)ranges. In one example, a sensor device may include a transmitting RFantenna to rotate a RF beam around the scan room, searching forpotentially MRI-incompatible objects in a manner similar to the laserprojector device described above. In this example, a receiving RFantenna may be placed on the floor around the perimeter of safety zone201, or around the shoe molding level of the surrounding walls to detectblockage any RF beams. The transmitting RF antenna of the sensor devicemounted on top of magnet housing 105 may operate in duplex mode toreceive RF beams reflected from objects in the scan room. Suchmonitoring approaches can be extended to monitoring using ultrasoundbeams. The monitored range information can be still another source ofinput data for further processing by the detection system 206, asdiscussed further below.

Some implementations may place a sensor device on MRI-incompatibleobjects in the scan room. For example, such a sensor device may beplaced on an anesthesia cart and configured to detect and transmitlocation data indicating a location of the cart. In some instances, thelocation data may include proximity data indicating the distance betweenthe sensor device on the object and sensor device mounted on magnethousing 105. Such location data may be communicated to the control unitof detection system 206. The communication between sensor devices andthe control unit may be through a wireless technology such as BlueTooth,infrared, WiFi, cellular, etc. In other instances, the location data mayalso indicate a motion or acceleration of cart 210 in the scan room.When acceleration motion is detected, one or more air bags may beinflated to buffer the potential impact of the accelerating objecthitting magnet housing 105 or patient 103 inside bore 101. The EMoperating frequency may be chosen to avoid aliasing with respect to theLarmor frequency, at which the MRI system 100 operates to generate an MRimage of patient 103. Such location data broadcast from sensor devicesmay be yet still another source of input data for further processing bythe detection system 206, as discussed further below.

Some configurations may leverage a magnetic readout. For example, sensordevices may be magnetic field probes placed on objects which can pose anoperational hazard within safety zone 201 (e.g. an anesthesia cart). Ifthe magnetic field probe detects that it is within a magnetic fieldabove a certain threshold, the field probe may send a signal, forexample, via radio wave or infrared light. Example magnetic field probesinclude Gauss meters or Hall probes. The readout signal may becommunicated to a control until, which may then analyze and compare thereadout value to a numerical threshold. If the readout value is indeedover the threshold safety value, a shutdown signal may be directed tothe power supply of the MRI system to trigger a shutdown of the magnet.The magnetic readout may also serve as an additional source of inputdata for further processing by detection system 206, as discussedfurther below.

Some implementations may exploit the inductive effect caused by amagnetic object that moves quickly through the magnetic field of themagnet. Specifically, such motions can induce fluctuations in thecurrent of superconducting wires powering the magnet. The effect is dueto inductive coupling between the object and the magnet of the MRIsystem 100. A sensor device may measure the level of currentfluctuation, which can then be compared to a threshold. If thefluctuation is above the threshold level, the control unit of detectionsystem 206 can trigger a shutdown sequence to turn down the power supplyof the magnet. The readout from this inductive approach may be anadditional source of input data for further processing by detectionsystem 206, as discussed further below.

The readouts from one or more of these various mechanisms may beconsolidated at the control unit of detection system 206 to render adetermination as to the nature of the object as well as the level ofthreat. In instances where more than one type of mechanism is used, thevarious readings may be used to corroborate each other to enhance theconfidence that a detected object poses a threat. For example, a readoutfrom a camera device may detect the presence of an object in the scanroom, while a readout from a magnetic field probe placed on the objectmay confirm that the detected object is seeing a magnetic field above athreshold level. Once a final determination is made based on thecombined readout that the detected object poses an operational hazard,precautionary steps may be taken by the control unit to turn down themagnet, including releasing an airbag to protect patient 103 beingscanned in bore area 101 and/or reducing the magnetic field of themagnet. In one illustrative example, a sensor device, such as a forcedevice or an accelerometer, may be mounted on magnet housing 105 todetect the impact of an object hitting magnet housing 105. If an impactis being detected and a human patient is being scanned in bore 101, oneor more air bag may be released to protect the patient.

In some implementations, turning down the magnet may not cause acontrolled quench of the magnet. Instead, in these implementations, themagnetic field of the magnet can be ramped up after the detected hazardis gone. For example, once the hazard is detected, the control unit ofMRI system may initiate automatic field reduction while signaling humanintervention. When it is determined that the operational hazard isremoved, for example, when cart 210 is removed from scan room by a humanoperator, a control unit of MRI system may initiate automatic ramping upto bring the magnet back to normal operation ready for scanningpatients.

For context, the wires that carry current are typically in asuperconducting state to power, for example, the solenoid magnet of theMRI system. In this superconducting state, the wires can carry largeamounts of current for generating a strong magnetic field of, forexample, 1 Telsa or above. Maintaining this superconducting stategenerally means that the wires are kept at a cryogenic temperature(e.g., below a certain critical temperature). As an illustration,temperature of the solenoid magnet is normally at 4 Kelvin. Typically ahelium compressor is used for maintaining the low temperature for thewires. If the helium compressor loses its alternate current (AC) mainpower (or the helium compressor is disconnected, or a cable damaged,etc.), the solenoid magnet will gradually warm up until the criticaltemperature of the superconducting wire is reached, at which point thewire would become resistive, and the resistive losses in the wire wouldcause a sudden increase of magnet temperature (also known as a quench)due to resistive heating and subsequently a precipitous drop in theamount of current carried by the wires. In this event, the magnettemperature rapidly rises to a much higher temperature such as 50Kelvin. The MRI system will be inoperable until the magnet temperatureis lowered back to, for example, 4K, which could take several hours oreven longer. To avoid this undesirable delay, the magnet temperature canbe monitored. If a loss of cooling is detected, the MRI system caninitiate a ramp down of the magnet. If the magnet has been ramped down(e.g., no current in the magnet wires) when the temperature crosses thesuperconducting critical temperature threshold, then there would be noquench, and no rapid increase in temperature. By adding this feature,the time for the magnet to be ramped up back for operation can bereduced.

FIG. 3 is a flow-chart 300 summarizing a process for safe shutdown of amagnet. After the process starts (302), a sensor device determineslocation information that indicates a location of an MR-incompatibleobject relative to the MRI system (304). Such determination may leveragea camera device configured to monitor the scan room for MR-incompatibleobjects. In the example of a camera device, the location informationdetermination may incorporate image processing of video frames capturedby the camera device. As discussed herein, the MRI system generates asubstantially uniform polarizing magnetic field for imaging a subject.Next, the process determines whether there is manual override (306).Manual override means a human operator is taking over the operation ofMRI system 100. If there is manual override, the MRI system 100continues operation under the guidance of the human operator (308). Ifthere is no manual override, then based on the determined locationinformation, a control unit associated with the MRI system determineswhether the MR-incompatible object poses an operational hazard to theMRI system (310). The operational hazard determination may includedetermining whether the MR-incompatible object is present within asafety zone of a main magnet of the MRI system. The operational hazarddetermination may be based on consolidated input from more than onesensor devices, not merely the camera device. In one instance, theoperational hazard determination may include a determination of whetherthe object is accelerating towards the main magnet. In response todetermining that the MR-incompatible object poses an operational hazardto the MRI system, the control unit may cause air bags to be inflated ora reduction of a strength of the substantially uniform polarizingmagnetic field (312). This reduction may include the initiation of ashutdown sequence for the magnet. When the operational hazard is nolonger a threat, for example, when cart 210 has been removed from scanroom, the control unit may cause a ramp-up of the strength of thesubstantially uniform polarizing magnetic field and normal operations ofthe MRI system may resume.

As used herein, the terms “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

As used herein, the term “exemplary” means “serving as an example,instance, or illustration,” and should not be construed as preferred oradvantageous over other configurations disclosed herein.

As used herein, the terms “about” and “approximately” are meant to covervariations that may exist in the upper and lower limits of the ranges ofvalues, such as variations in properties, parameters, and dimensions. Inone non-limiting example, the terms “about” and “approximately” meanplus or minus 10 percent or less.

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover all modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

What is claimed is:
 1. A magnetic resonance imaging (MRI) system,comprising: a housing having a bore in which a subject to be imaged isplaced; a main magnet accommodated by said housing and configured togenerate a polarizing magnet field with the bore; pulse generating coilsconfigured to generate and apply radio frequency (RF) pulses in sequenceto scan the subject; gradient coils configured to provide perturbationsto the polarizing magnet field that encode MRI signals acquired inresponse to the applied RF pulses; and a control unit coupled to themain magnet and configured to: access location information thatindicates a location of an MR-incompatible object relative to the MRIsystem; based on the location information, determine whether theMR-incompatible object poses an operational hazard to the MRI system;and in response to determining that the MR-incompatible object poses anoperational hazard to the MRI system, reduce a strength of thepolarizing magnetic field.
 2. The MRI system of claim 1, wherein thepolarizing magnetic field is substantially uniform.
 3. The MRI system ofclaim 1, wherein the control unit is configured to receive locationinformation of the MR-incompatible object from a sensor mounted on theMR-incompatible object.
 4. The MRI system of claim 3, wherein the sensorcomprises one of: a Gauss meter, a Hall probe, a magnetic field probe,or an optical tracker.
 5. The MRI system of claim 1 further comprising acamera, wherein the control unit is configured to process images fromthe camera to determine the location information of the MR-incompatibleobject.
 6. The MRI system of claim 1 further comprising a currentsensor, wherein the control unit is configured to process a signal fromthe current sensor to detect a change in a current that powers the mainmagnet, the change being induced by a motion of the MR-incompatibleobject within the polarizing magnetic field.
 7. The MRI system of claim1, wherein, to reduce the strength of the polarizing magnetic field, thecontrol unit is configured to initiate a shutdown of the main magnet.